1. Field of the Invention
Embodiments of the present invention relate in general to optical pickup apparatuses and methods. More specifically, embodiments of the present invention relate to an optical pickup apparatus and method capable of recording and/or reproducing information onto and/or from optical discs having diverse specifications, simply by changing a diffraction angle of a laser diode, according to the wavelength of the laser diode used and the optical disc specifications.
2. Description of the Related Art
In general, optical pickup apparatuses are employed in CD players (CDPs), DVD players (DVDPs), CD-ROM drivers, and DVD-ROM drivers, to record and/or reproduce information onto and/or from a non-contact type recording medium.
The DVDP and the DVD-ROM, for example, are capable of recording and reproducing high-density data, so they have been preferably used with video and audio products. Particularly, the optical pickup apparatus for use in the DVDP is capable of recording and/or reproducing information on and/or from a DVD-ROM, and is manufactured to be compatible with the CD family media such as CD, CD-R (Recordable), CD-I, and CD-G, for example.
However, the thickness of DVDs has been standardized differently from that of the CD family media, at least in consideration of the allowable error in tilt angle and objective lens numeral aperture (NA). That is, the standard thickness of existing CD family media is 1.2 mm, whereas the standard thickness of DVDs is 0.6 mm. As such, when an optical pickup apparatus for DVDs is used for the CD family media spherical aberrations occur due to the difference in thicknesses between the two types of discs. As a result of the spherical aberrations, sufficient light intensity cannot be provided for recording an information signal or the signal reproduced from the CD is deteriorated.
In addition, the track pitch of a DVD is at least two times denser than that of a CD, and the spacing between each pit, for example, is much narrower, so the DVD storage capacity is several times larger than that of the CD storage capacity. As CD and DVDs have different thicknesses and storage capacities, wavelengths of laser beams used are different. For instance, the CD uses an infrared beam having a 780 nm wavelength for recording and/or reproducing information, while the DVD uses a red light beam of a 635 nm wavelength.
As described above, wavelengths of lasers used for the DVD and the CD are different because the DVD and the CD have different thicknesses and different track pitches. Therefore, it is difficult to reproduce information from the DVD and the CD using one optical system. In an attempt to solve this problem, lens switching methods have been developed. According to one lens switching method, two objective lenses, one for the CD and the other for the DVD, are installed. Even though this makes the product design more complicated and increases the cost of manufacture, recently it has been the best known method for use with the DVD and the CD.
Most recently, a new method has been introduced, where information is read from two media using only one DVD lens, with NA and annular control systems being typical examples thereof. Since these two systems use only one lens, they are cost effective and simplify the product design. Furthermore a hologram using two focal points with one DVD lens or a non-spherical lens corresponding to the NAs of two media can be used also.
Meanwhile, optical pickup apparatuses, depending on the type of light receiving element (or photodiode) used, can be divided into optical pickup apparatuses based on a push-pull method using a 4-split photodiode, and optical pickup apparatuses based on a 3-beam method using an 8-split photodiode. The 3-beam optical pickup apparatus is generally used for high-density discs where an accurate tracking servo performance is highly appreciated.
As can be seen in FIG. 1, the 3-beam optical pickup apparatus includes a LD (Laser Diode) 1 for emitting two light beams of different wavelengths, a diffraction grating 2 for splitting a laser beam from the LD 1 into three discrete beams, namely one main beam and two sub beams, a beam splitter 3 for reflecting a beam off of the diffraction grating 2 towards an optical disc and transmitting a reflected beam from the optical disc, a collimator lens 4 for condensing parallel beams reflected from the beam splitter 3 to form a beam spot on an information recording surface of the optical disc, and transforming a divergent beam of the beam spot reflected from the information recording surface of the optical disc to a parallel beam (or collimated beam), an objective lens 5 for condensing the reflected light transmitted through the beam splitter 3, and a photo detector 6 for photoelectrically converting three beam spots condensed by the objective lens 5 into electric signals.
Depending on whether the information recording medium is a CD or a DVD, the LD 1 of the optical pick up apparatus outputs laser beams having different wavelengths. An output laser beam is then split by the diffraction grating 2 into a main beam and sub beams. In other words, the diffraction grating 2 reads an information signal from the laser beam output from the LD 1, and splits the laser beam into an 0th-order main beam for a focus servo of the optical disc and ±1st-order sub beams for a track servo of the optical disc.
The split beams are reflected towards the optical disc by the beam splitter 3, and focused onto the optical disc by the collimator lens 4 to form beam spots, and reflected again from the optical disc. The reflected beams from optical disc transmit through the beam splitter 3, as collected by the objective lens 5, and detected by the photo detector 6. By detecting a main beam focused on a main beam spot part 6a, the photo detector 6 reads out stored information of the beam and, at the same time, detects focusing signal error of the optical disc. The photo detector 6 detects a tracking signal error of the optical disc by using a sub beam focused on a sub beam spot part 6b. 
The beams (i.e., the main beam and the sub beams) are split through the diffraction grating 2 and reflected towards the optical disc, and focused onto the optical disc by the collimator lens 4. As shown in FIG. 2, the main beam is incident on a central part, and the pair of sub beams are incident on alternate sides. The main beam should be incident accurately on the track because it should read the recorded information signal. In addition, a straight line connecting each sub beam to the main beam and the track of the optical disc are at an angle of θ. At this time, the angle θ is determined based on the tracking method.
In one case of following the track using the diffraction grating, as shown in Table 1 below, a DVD-RAM has a track pitch of 1.23 μm, a DVD-RW has a track pitch of 0.74 μm, and a CD has a track pitch of 1.60 μm. In addition to the track pitch, the track servo control method differs. As a result, the angle θ between the straight line connecting each sub beam to the main beam and the track of the optical disc differs also. For instance, the angle θ for the DVD-RAM is 2.49°, whereas the angle θ for the DVD-RW is 1.50° and the angle θ for the CD is 2.71°. These angle requirements should be met for each sub beam to be incident upon a position within a measure of the track servo method. Therefore, it is very important that the main beam and the sub beams are incident on each optical disc at a corresponding angle θ with the track. To meet the angle requirement, the diffraction grating itself was rotated to adjust the incidence positions of the main beam and the sub beams upon the optical disc, or the tracking method itself was changed.
TABLE 1BD (BlueRay Disc)DVD-RAMDVD-RWCDWavelength (nm)408658658785Focal length of20202020collimating lens (mm)Focal length of2.22.282.282.28objective lens (mm)Track pitch (μm)0.321.230.741.60Diffraction20202020grating pitch (μm)Rotation angle1.08°2.49°1.50°2.71°of diffractiongrating (deg)Track servo methodDPPDPPDPP3-BEAM
Recent technical advances in the compact disc industry have introduced a BD (Blue ray Disc) with a capacity of 25 Gbytes, and there is a very high possibility that additional new discs with different specifications and greater capacities may be developed in the near future. Unfortunately though, the track pitch of the BD is 0.32 μm, which is only half of the track pitch of existing DVD-RWs, and the angle θ between the straight line connecting each sub beam to the main beam and the track of the optical disc is 1.08° degrees. Therefore, for tracking the BD, rotating the diffraction grating adjustively to the BD would be the best way. However, the rotation angle of the diffraction grating is set for the existing CDs and DVDs, so there is a limit to the rotation angle. This means that the range of the rotation angle of the diffraction grating has to be changed again adjusting for the BD. Further, a new optical system will have to be developed or an additional optical system will have to be added to the existing optical systems in order to reproduce information from BDs or discs having additional specifications that might be developed in near future. In so doing, the structure of the optical pickup apparatus, thus, has become complicated, increasing manufacturing costs.
Therefore, there is a need for an optical pickup apparatus having a diffraction grating compatible with existing CDs and DVDs as well as other discs with different specifications that might be developed in future.